blender 2.8 flat color for 2d animation
You can directly use color in surface socket

The color will show up directly in render mode (both Render and DevLook)

And for alpha channel, the original alpha in RGB mode can't be the value in surface(might be a bug?) .An alternative way is use transparent shader with mix shader instead as shown below:




the factor of mix shader can be the alpha value.
Blend mode in Material setting should set to one of the alpha options
Show backface will cause some problem, toggle it to see which one suit the scene


There might be a better way on this task